Magnetic proximity switches, also known as limit switches, are commonly used for position sensing. Typically, a magnetic proximity switch assembly includes a target and a proximity switch, with the proximity switch including a switching circuit. The switching circuit may include an element, such as a lever, that is biased in a first position by a permanent magnet contained in the housing of the proximity switch. With the lever in this first position, the proximity switch is maintained in a first state, in which, for example, a normally closed contact makes contact with a common contact. When the target, which generally includes a permanent magnet, passes within a predetermined range of the proximity switch, the magnetic flux generated by the target magnet causes the lever of the switching circuit to change bias from the first state to a second state, in which, for example, a normally open contact makes contact with the common contact.
In some applications, one or more target magnets and one or more proximity switches may be disposed within a sealed enclosure to protect the proximity switches from damage. This configuration is common when the magnetic proximity switch assembly is used in hazardous environments, such as nuclear applications. In such applications, the enclosure is intended to withstand the high temperatures and pressures that occur during a containment accident or a LOCA (loss of coolant accident) at a nuclear facility. Typically, a shaft vertically disposed within the enclosure supports the target magnet as the target magnet rotates with the shaft relative to a stationary proximity switch. Typically, a top portion of the shaft is coupled to a sealed top bearing assembly disposed within a top aperture that extends through a top portion of the enclosure, and a bottom portion of the shaft is received in a bottom aperture that extends through a bottom portion of the enclosure. The bottom portion of the shaft that extends through the bottom aperture is typically coupled to a valve element, such as the rotating stem of a control valve used for a nuclear application, and the rotation of the stem can be detected with the valve rotates the target magnet within a predetermined range of the proximity switch disposed within the enclosure, thereby indicating the control valve is in a particular position. Alternatively, the rotating stem of the control valve may move the target magnet out of a predetermined range of the proximity switch, thereby indicating the control valve has moved from a particular position.
Due to the hazardous environments in which the magnetic proximity switch assembly is used, the enclosure must be sealed to avoid allowing high temperature gas under high pressure, or other contaminants, into the enclosure.